Multi-layer electrically isolated thermal conduction structure for a circuit board assembly

ABSTRACT

A thermal conduction structure is integrated into a multi-layer circuit board to thermally couple a power electronic device to a heatsink while electrically isolating the power electronic device from the heatsink. The thermal conduction structure includes a stack of alternatingly insulative and conductive layers, a first set of vertical vias that thermally and electrically join the power electronic device to a first set of conductive layers, and a second set of vertical vias that thermally and electrically join the heatsink to a second set of conductive layers that are interleaved with the first set of conductive layers.

TECHNICAL FIELD

The present invention relates to an electronic circuit board assemblyincluding a power electronic device mounted on a multi-layer circuitboard, and more particularly to an electrically isolated thermalconduction structure integrated into the multi-layer circuit board fordissipating heat generated by the power electronic device.

BACKGROUND OF THE INVENTION

Various thermal conduction structures have been used to dissipate heatgenerated by the power electronic devices of electronic circuit boardassemblies. A common approach is to attach a heatsink to the circuitboard substrate opposite the power electronic devices, and toincorporate thermal vias into the circuit board for thermally couplingthe power devices to the heatsink. While the thermal via approach can beeffectively used in certain applications, it is frequently necessary toelectrically isolate the power electronic devices from the heatsink bydepositing a layer of special insulating material between the circuitboard and the heatsink. This not only increases the cost of the circuitboard assembly, but also reduces thermal coupling of the circuit boardto the heatsink. Accordingly, what is needed is a circuit board havingan electrically isolated thermal conduction structure that isinexpensive to manufacture and that provides improved thermal coupling.

SUMMARY OF THE INVENTION

The present invention is directed to an improved thermal conductionstructure integrated into a multi-layer circuit board for thermallycoupling a power electronic device to a heatsink while providingelectrical isolation between the power electronic device and theheatsink. The thermal conduction structure includes a stack ofalternatingly insulative and conductive layers, a first set of verticalvias that thermally and electrically join the power electronic device toa first set of conductive layers, and a second set of vertical vias thatthermally and electrically join the heatsink to a second set ofconductive layers that are interleaved with the first set of conductivelayers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top view of an electronic circuit board assembly includinga power electronic device and a multi-layer circuit board incorporatinga thermal conduction structure according to this invention;

FIG. 1B is a cross-sectional view of the an electronic circuit boardassembly of FIG. 1A taken along lines 1B-1B of FIG. 1A; and

FIG. 2 is an isometric illustration of a thermal conduction structureaccording to this invention, with core insulative layers of thestructure removed to reveal the thermal conduction paths.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In general, the present invention is directed to a multi-layer thermalconduction structure designed into a multi-layer circuit board orsubstrate. A power electronic device is mounted on one side of thethermal conduction structure, and a heatsink is affixed to the otherside of the thermal conduction structure. The purpose of the thermalconduction structure is to thermally couple the power electronic deviceto the heatsink while electrically isolating the power electronic devicefrom the heatsink. The thermal conduction structure is illustrated inthe context of a multi-layer woven glass (FR4) circuit board, but itwill be appreciated that the structure could alternately be implementedin a multi-layer ceramic circuit board.

Referring to FIGS. 1A-1B, the reference numeral 10 generally designatesan electronic circuit board assembly incorporating a multi-layer thermalconduction structure 12 according to this invention. Thermal conductionstructure 12 is designed into a multi-layer circuit board 14 (alsoreferred to herein as a substrate) of conventional construction,resulting in exposed upper and lower copper pads 16 and 18. A powerelectronic device 20 having an exposed metal base 22 is soldered orotherwise thermally affixed to a central region of the upper copper pad16, and the pedestal 24 of a metal heatsink 26 is soldered or otherwisethermally affixed to a central region the lower copper pad 18 oppositethe device 20. For example, a thin layer 25 of solder may be used toprovide a high thermal conductivity joint between copper pad 18 andpedestal 24. In the illustration, the metal base 22 forms one terminalof the device 20, and the other terminals 28 and 30 of device 20 extendbeyond the perimeter of thermal conduction structure 12 where they aresoldered to respective pads 32 and 34.

As depicted in the cross-sectional view of FIG. 1B, the thermalconduction structure 12 comprises a number of alternatingly insulativeand conductive layers between the upper and lower copper pads 16 and 18that insulate upper copper pad 16 from lower copper pad 18 whileproviding thermal conduction between adjacent layers to transfer heatdissipated by the device 20 to the metal pedestal 24. The referencenumerals 40-52 designate the insulative layers, while the referencenumerals 54-64 designate the conductive layers. Insulative layers 40-44(also referred to herein as core layers) and conductive layers 54-64 aredefined by a three-sheet stack of copper-clad epoxy-impregnated wovenglass circuit board material such as FR4. That is, conductive layers 54and 56 are bonded to opposing faces of the top core layer 40; conductivelayers 58 and 60 are bonded to opposing faces of the middle core layer42; and conductive layers 62 and 64 are bonded to opposing faces of thebottom core layer 44. Typically, the core layers 40-44 will each have athickness of 0.010 inch, for example, and the copper layers 54-64 willeach have a thickness of 0.0028 inch, for example. The insulative layers46-52 electrically isolate adjacent conductive layers 16, 54-64 and 18.Specifically, insulative layer 46 is disposed between upper copper pad16 and conductive layer 54; insulative layer 48 is disposed betweenconductive layers 56 and 58; insulative layer 50 is disposed betweenconductive layers 60 and 62; and insulative layer 52 is disposed betweenconductive layer 64 and lower copper pad 18. The insulative layers 46-52are preferably formed of pre-impregnated woven glass material (i.e.,pre-preg FR4) having a thickness of 0.003 inch, for example, and serveas a thermal bridge between respective adjacent conductive layers.

Thermal conduction structure 12 additionally includes an array ofcopper-lined vias 66 and 68 that are located in a marginal area beyondthe footprint of the device 20, and that extend vertically through theentire structure. The vias 66 are joined to the upper copper pad 16 andconductive layers 56, 60 and 64, but are separated from the lower copperpad 18 and conductive layers 54, 58 and 62. On the other hand, the vias68 are separated from upper copper pad 16 and conductive layers 56, 60and 64, but are joined to lower copper pad 18 and conductive layers 54,58 and 62. This preserves the electrical isolation between upper andlower pads 16 and 18, while providing vertical thermal conduction pathsbetween alternating conductive layers.

It will be appreciated that the above-described thermal conductionstructure 12 can be designed into (and replicated in) a multi-layercircuit board using conventional circuit board manufacturing techniquessuch as photo-etching the copper layers bonded to core layers 40-44,drilling vertically through the layered assembly to locate the vias66-68, and copper-plating the drilled openings to form the vias 66-68.For example, the conductive layers 54, 58, 62 and 18 are circularlyetched to ensure that they remain electrically isolated from the vias66. Similarly, the conductive layers 16, 56, 60 and 64 are circularlyetched to ensure that they remain electrically isolated from the vias68, as seen in the illustration of FIG. 2 where the core layers 40-44have been omitted to reveal the primary thermal conduction paths betweenupper and lower pads 16 and 18.

In summary, the thermal conduction structure 12 of present inventionprovides a reliable and cost-effective way of transferring heat from apower electronic device to a heatsink while electrically isolating thepower electronic device from the heatsink. Heat applied to upper pad 16by the device 20 is successively transferred to the intermediate copperlayers 54-64 and 18 through the thin insulative thermal bridge layers46-52 and the vias 66-68, and is then transferred to the metal pedestal24 through the solder or conductive adhesive layer 25 joining lower pad18 to heatsink 26. Testing has indicated that the described thermalconduction structure 12 thermally outperforms known via structures,while avoiding the expense associated with providing a specialinsulative interface between the circuit board and the heatsink.

While the invention has been described with respect to the illustratedembodiment, it is recognized that numerous modifications and variationsin addition to those mentioned herein will occur to those skilled in theart. For example, the shape, thickness, number of layers and materialcomposition may be different than shown herein, one or more blind viasmay be included in the structure, and so on. Accordingly, it is intendedthat the invention not be limited to the disclosed embodiment, but thatit have the full scope permitted by the language of the followingclaims.

1. A multi-layer electrically isolated thermal conduction structure forthermally coupling a power electronic device mounted on a first side ofthe structure to heatsink thermally coupled to a second side of thestructure, comprising: a stack of alternatingly insulative andconductive layers disposed between the power electronic device and theheatsink, including a first set of conductive layers thermally andelectrically coupled to the power electronic device, and a second set ofconductive layers thermally and electrically coupled to the heatsink,the second set of conductive layers being interleaved with the first setof conductive layers but electrically isolated from the first set ofconductive layers and the power electronic device by the insulativelayers.
 2. The multi-layer electrically isolated thermal conductionstructure of claim 1, further comprising: a first set of conductive viasthat thermally and electrically join the first set of conductive layersto the power electronic device; and a second set of conductive vias thatthermally and electrically join the second set of conductive layers tothe heatsink.
 3. The multi-layer electrically isolated thermalconduction structure of claim 2, where: the first set of conductive viasis electrically isolated from the second set of conductive layers; andthe second set of conductive vias is electrically isolated from thefirst set of conductive layers.
 4. The multi-layer electrically isolatedthermal conduction structure of claim 1, where: the stack ofalternatingly insulative and conductive layers are designed into amulti-layer circuit board.
 5. The multi-layer electrically isolatedthermal conduction structure of claim 1, where: the insulative layersinclude a set of core layers to which the conductive layers are affixed,and a set of thermal bridge layers disposed between adjacently disposedconductive layers.
 6. The multi-layer electrically isolated thermalconduction structure of claim 5, where: the core layers are formed ofepoxy impregnated woven glass material; and the thermal bridge layersare formed of pre-impregnated woven glass material.
 7. The multi-layerelectrically isolated thermal conduction structure of claim 6, where:the thermal bridge layers each have a thickness of approximately0.003-0.004 inch.
 8. The multi-layer electrically isolated thermalconduction structure of claim 1, further comprising: a solder layerthermally coupling the heatsink to the second side of the structure.